The present disclosure relates to a portable forensic light source device for mobile use at a crime scene.
In forensic criminology, so-called forensic lighting devices are used for crime scene investigation. They do not only serve for illumination of poorly lit crime scenes but also for visualizing evidence traces, such as hair, body fluids or fingerprints, at the crime scene by means of fluorescent effects.
For this latter use, forensic lighting devices are required to be capable of emitting not only intense white light but also light of particular spectral regions within the range of 300-700 nm wavelength with sufficiently high emission power, such that fluorescent effects may be observed also at daylight. The so-called forensic range of use comprises the frequency range useful for crime scene investigation and ranges from ultraviolet light (UVA-light) of a wavelength of circa 300 nm up to the long wave limit of visible light at a wavelength of circa 700 nm. Here the bands UVA (320-400 nm), Blue (400-500 nm) and Green (500-560 nm) are of particular importance, as they are used for exciting fluorescence of traces, often in connection with coloring methods. The long wave fluorescence emission caused by the Stokes shift is observed through observation glasses equipped with longpass filter lenses blocking short wave excitement emission.
For allowing the light sources to be used as mobile and flexible as possible, it is desired to operate them with an accumulator independent from the line current. In order to improve the handling at the scene, forensic lighting devices further comprise liquid core light guides attached to the light source device guiding the light from the light source inside the device to the light output in the hand of the user.
In order to make shoeprints visible, so-called beam cross-section converters are often used. These are usually formed by a one-sided linearly expanded bundle of optical fibers operating as a flexible light guide from the light source to the floor where light is emitted tangentially onto a wider area, such that shoeprints may be detected more readily.
Xenon super high pressure lamps of sufficiently high electrical power (300 to 500 W) offer a high illumination power continuously over the visible as well as over the non-visible spectral range. However, two thirds of the emission energy are in the near infrared range and therefore outside the forensic range of use. As a high power supply is required due to the low level of efficiency, it has to be provided by the power supply system or a particularly powerful and hence heavy accumulator. The resulting heavy weight of the crime scene light source including the accumulator would amount to more than 30 kg, such that the device would be bulky and too heavy to carry. Consequently, xenon forensic lighting devices in this power range (300-500 W) are not available at all with accumulators. Moreover, the high infrared portion of the emitted light results in an excessive heating of the filter and the liquid core light guide, thereby shortening the light guide's lifetime. Furthermore, the strong heat generation requires the use of light guides with relatively large light-active diameters (8 to 10 mm). This limits flexibility and ease of use. Xenon lamps with less power (e.g. circa 35 W) do not suffer from the above mentioned disadvantages, but exhibit an emission power in the range of use which, in particular in the near UVA range, is much too poor, such that at daylight crime scene investigation by fluorescence excitation becomes impossible.
Mercury super high pressure lamps such as the Osram HBO lamp with a mercury pressure of less than approx. 107 Pa (ca. 100 atm) exhibit a pronounced line spectrum. Due to the lack of a continuous spectral background between the spectral lines in the range of wavelengths between 300 and 700 nm, the possibility of selecting particular spectral windows is strongly restricted, as it is basically confined to the position of the spectral lines of mercury. Moreover, the long heating time after switching on the forensic lighting device (some minutes) exclude a quick availability.
Tungsten halogen lamps emit 90% of their radiation within the infrared range, whereas the UVA portion is negligibly small. Therefore, the efficiency is even smaller than that of xenon lamps. While operation with an accumulator is possible up to a power of 150 W, the available radiation in the important range of UVA, blue and green is extremely small.